Optimization of long-period grating-based refractive index sensor by bent-fiber interference.

In this paper, we propose and demonstrate a novel approach to enhance the refractive index (RI) sensitivity and eliminate the temperature cross-sensitivity of a long-period grating (LPG) -based refractive index sensor by bent-fiber interference. The approach is based on a hybrid structure composed of an LPG and a bent-fiber intermodal interferometer. The bent-fiber intermodal interferometer has a simple structure, which consists of a bare fiber semi-circular bending region with a 5 mm bending radius. As the RI increases, the resonance wavelength of the LPG moves toward a shorter wavelength, while the resonance wavelength of the bent-fiber intermodal interferometer shifts to a longer wavelength. The separation of two resonance dips increases with the RI; using two resonance dips allows us to measure an RI with a higher sensitivity than if we had only used one resonance dip. However, as the temperature increases, the separation of the two resonance dips is constant. This approach can effectively enhance the RI sensitivity and eliminate temperature cross-sensitivity.